Hitherto, porous membranes of vinylidene fluoride resin excellent in weatherability, chemical resistance, heat resistance, strength, etc., have been used as microfiltration membranes for drugs, bacteria, etc., or as separators of batteries.
For the production of such porous membranes of vinylidene fluoride resin, there have been proposed, e.g., a process of mixing a vinylidene fluoride resin with an organic liquid such as diethyl phthalate and hydrophobic silica as an inorganic powder, melt-forming the mixture, and then extracting the organic liquid and silica (JP-A 3-215535), a process of forming a vinylidene fluoride resin into a porous membrane through crystallization, heat treatment, stretching and heat treatment under tension, under respectively controlled conditions (JP-A 54-62273), a process of forming a film of vinylidene fluoride resin of a specific molecular weight together with a plasticizer, cooling the film from one side thereof and then extracting the plasticizer (JP-A 7-13323), and a process of mixing a vinylidene fluoride resin of an ordinary molecular weight with a high-molecular weight vinylidene fluoride resin for increasing the heat distortion resistance and an organic pore-forming agent or an inorganic pore-forming agent to form a film, and then extracting the pore-forming agent for removal or using the inorganic pore-forming agent as nuclei of stress concentration at the time of stretching to form pores in the film, thereby forming a porous membrane (JP-A 2000-309672).
The thus-formed porous membrane of vinylidene fluoride resin is excellent in weatherability, chemical resistance, heat resistance, strength, etc., as mentioned above, but when it is used as a water-treating membrane for cleaning, it is required to prevent the soiling (fouling) with an organic matter such as protein, thereby preventing a lowering in water permeation rate for a long period. For this purpose, it has been proposed to hydophilize the porous membrane of vinylidene fluoride resin. More specifically, for the hyophilization, there have been proposed a method of coating the surface of a hollow fiber membrane with ethylene-vinyl alcohol copolymer (JP-A 2002-233739) and a method of forming a polyvinylpyrrolidone crosslinked product on a wet membrane (JP-A 11-302438).
However, such prior art methods are accompanied with difficulties, such as (1) coating or crosslinking operation is required to increase the number of steps, (2) as a result of the coating or crosslinked product, the original pore structure is changed, thereby making it difficult to control the water permeability, and the initial water permeation rate is remarkably lowered due to the treatment, and (3) it becomes difficult to take advantage of chemical resistances, such as anti-ozone and anti-chlorine properties, which are inherently owned by the vinylidene fluoride resin.